Sunday, November 17, 2013


I really ought to start documenting projects as I go along. 

Broken down into three parts, yo.

1. Lasers!

Once on my bucket list, this blue laser pointer was what constituted an overnight build-a-thon at MITERS. In my laser-induced, sleep-deprived stupor, I forgot to document much of the build, although, there wasn't much to do to begin with.

Diode: 445nm rated for 200mW
Power: two lithium cells (7.2V nominal)
Driver: ebay diode driver (good for stuffing into small spaces)

Given that the laser diode has a TO-18 package, I had to make and press-fit the heatsink into a brass adapter ring to fit in the lens assembly. Some patience on the lathe and a bit of sandpaper made the task at hand much easier. This was by far the hardest task.

The shaft was machined out of some aluminum stock and the end cap is mounted to the body with a set screw that isn't actually a set screw for convenience's sake. The shaft was press fit into the threaded lens/diode assembly.

Setting the power output of the diode driver:

I assembled a dummy load from a blue LED and a 10 ohm resistor and then measured the voltage drop across the resistor, which gave me the current output. The diode is set to run comfortably at 40mA to give me a 40mW output. I later adjusted this down to 8mW since I found the output even at 40mA too bright to use in any useful setting (powerpoint presentations, pointing at stuff safely).

*NOTE: the TTL pin has to be tied to Vcc in order for the potentiometer to function; otherwise, the diode driver will only give output at 0mA and 500mA and nothing in between. Also, the potentiometer is continuous, so be wary of adjusting the thing once it's soldered to the diode.

Thanks to Bayley Wang for the parts/instructions.

Diagram of the construction:

One of the more interesting aspects of having a laser of such a funny wavelength is that you can cause things to fluoresce  in the visible spectrum.

suddenly: violet!
It's rather unfortunate that photos can't do justice to its wonderful luminescence.

2. A wild speaker appears!

Yet another school project =_____=. Groups were given some magnets, washers, and bolts. We were then given the directive to build a functional speaker. Promising, I know.

Major difficulties: quantitative analysis of such a rudimentary system is essentially futile. I tried anyway. Kinda.

The requisite flat frequency response was to occur from 100Hz to 20kHz, which led me to try and get a mechanical resonance at slightly 100Hz. This would allow me to add a port tuned to the appropriate frequency to extend the bass range while keeping the high frequency response that I wanted.

In addition to having a flat response, this thing had to be reasonably loud; given that the speaker would have an impedance matched to the source, this meant I had a fixed length of wire: increasing the inductance would allow for more force to be applied to the coil as indicated by the Lorentz Force Law. This meant keeping the coil reasonably short and fat to increase inductance, as well as double layering the coil.

The tradeoff was that at a higher frequencies, the reactance could get as high as 4 ohms, however, that would only result in a 3% reduction in power, which was reasonable.

The choice for enclosure was acrylic due to its high compressive yield strength and ease of manufacture (laser cutter access). It also looks pretty.

Box volume was limited by the quantity of acrylic I had at hand - in good speaker design, the volume is supposed to be matched to the volume of air that the speaker displaces, its resonant frequency, as well as its mechanical and electrical Q. Again, quantities that are hard to measure. I ended up doing the thing I shouldn't do, which is stuff the box full of cotton to increase its apparent volume and the remove it until it seemed to sound best. Miraculously, the box without any additions seemed to work.

The port was also hand tuned to aroud 90Hz, which involved using an exacto knife to slowly cut away at its length (it ended up being ~1.2" in length).

Magnet arrangement was chosen mainly to achieve symmetry, but at the same time, maintain the flux density required to give the speaker enough output.

If there were one thing I'd do over in this speaker, it would be getting stronger magnets and reducing the inductance of the voice coil to maintain good output, but keep a nice high frequency response.

Lastly, the membrane, which provides the restorative force to the moving cone (made of a manila folder, no less),  was provide by some polyurethane sheet stretched and then hot glued against the frame of the speaker. This formed some semblance of a surround. The thought behind using such a lightweight material is to reduce the apparent mass that the coil has to move, which would otherwise attenuate higher frequencies.

The end result:

3. More coil things!

As part of a final project, I was granted funding for another coil. The nuance is that I'll have to make an analog interrupter that goes with it.

This will involve a revamp of Derpy coil into a separate driver and bridge to keep board costs down.

The interrupter is just a VCO made out of a 555 timer and an op amp fed with the audio. Nothing too fancy.

Current iteration of the driver
OCD has been added in the schematic, but has yet to be laid out. Just think of it as a UD, but without the totem pole driver.

More to come...